1. Field of the Invention
The invention is related to the field of mass flowmeter systems, and in particular, to a method of measuring propane using a mass flow rate from a Coriolis flowmeter.
2. Statement of the Problem
Approximately 36,000 bobtail trucks operate on United States highways and in airports to deliver various fuels. A sale or custodial transfer typically occurs whenever fuels are offloaded from these trucks or trailers. Typically, positive displacement metering devices are installed to measure volumes of materials that are offloaded from these trucks.
At present, only positive displacement meters have obtained government approval for fuel delivery sales. Other types of meters have either not been tested to ascertain whether they are capable of withstanding the rigors of roadable use or they have commonly failed under conditions of such use.
Bobtail trucks are used to transport liquid petroleum gas or LPG products. The most common form of LPG is propane. Propane results from the process of refining crude oil, as well as the processing of natural gas. Propane and other forms of LPG are often stored underground in salt domes, anticlines, and other geologic formations until they are needed. Bobtail trucks are used to transport the LPG to remote points of use, e.g., to rural houses having propane tanks where the propane is used for heating, automotive propane fueling stations, or to neighborhood distribution points for filling propane tanks that are used in residential barbeques and the like. Propane is preferred because it becomes liquid at low pressures, which are typically less than 150 psia. Unfractionated natural gas may also be used in these devices, but natural gas requires storage at significantly greater pressures which can result in catastrophic failure of metal storage vessels.
A problem arises in these sales due to variations in volume that derive from different temperatures and pressures. When measured sales volumes are corrected for these effects, it is most often the case that only temperature effects are corrected. Temperature correction is normally accomplished by using standard published data from the American Petroleum Institute, e.g., Table 24 Volume Reduction to 60xc2x0 F., Petroleum Measurement Tables: API Standard: 2450 (ASTM Designation D:1250) 1952 American Ed.
Despite common practice of correcting only for temperature on the assumption that liquids are essentially incompressible at low pressures around 150 psi, LPG remains highly compressible even in liquid form at these pressures. Conventional positive displacement meters are insensitive to changes in pressure and temperature that affect measurement accuracy relative to standard conditions, e.g., at 60xc2x0 F. and 150 psia for propane. Conventional positive displacement meters can measure a volume of displaced liquid, but there is insufficient information available to convert the volume to a standard volume, i.e., a corresponding volume at standard pressure and temperature conditions.
Positive displacement meters often break or fail under field conditions. For example, propane is a notoriously poor lubricant, and the positive displacement meters that are used to deliver propane simply wear out quickly as a consequence of poor lubrication in the intended environment of use. Particles in the materials being delivered can jam the moving parts of positive displacement meters, which then demonstrate measurement error. Furthermore, the positive displacement meters are insensitive to changes in pressure, temperature, and fluid density in the materials being delivered. These conditions combine to provide an unacceptably high meter uncertainty in the field.
Mass flowmeters are not often used in these applications, in part, because they have not yet obtained the requisite regulatory approvals. Another reason why mass flowmeters have not been used in this intended environment of use is the fact that they measure mass, as opposed to volume, where the sales must take place in terms of volume. Some types of mass flowmeters, especially Coriolis flowmeters are capable of being operated in a manner that performs a direct measurement of density, and volume is obtainable as the quotient of mass over density. For example, U.S. Pat. No. 4,872,351 describes a net oil computer that uses a Coriolis flowmeter to measure the density of an unknown multiphase fluid. U.S. Pat. No. 5,687,100 describes a Coriolis-effect densitometer that corrects the density readings for mass flow rate effects in a mass flowmeter operating as a vibrating tube densitometer.
Coriolis flowmeters measure mass flow and other information for materials flowing through a conduit. Such flowmeters are disclosed in U.S. Pat. Nos. 4,109,524, 4,491,025, and Re. 31,450. Coriolis flowmeters have one or more flow tubes of a straight or curved configuration. Information regarding the characteristics of material flowing in a Coriolis mass flowmeter must be derived with great accuracy because it is often a requirement that the derived flow rate information have an error of less than 0.15%.
Coriolis flowmeter output signals are sinusoidal and are displaced in time or phase by an amount determined by the Coriolis forces that are generated by the flowmeter through which the material flows. The signal processing circuitry which receives these sensor output signals measures this time difference with precision and generates the desired characteristics of the flowing process material to the required error of less than 0.15%.
Agencies that regulate the sale and delivery of propane require the delivering entity to provide both a gross volume and an adjusted volume (which may also be referred to as a net volume). The regulatory agency uses the gross volume to test the accuracy of the flowmeter and grant or deny approval of the flowmeter based on the accuracy.
When delivering to a customer, the flowmeter system, such as a positive displacement meter, measures and provides a gross volume. The flowmeter system then adjusts the gross volume based on one or more factors to get the adjusted volume. Unfortunately, if the calibration of the flowmeter is off or the flowmeter is damaged, the error produced by the flowmeter becomes compounded when the adjusted volume is calculated based on the gross volume. This could result in unacceptable errors in the volume measurements of propane.
The invention helps solve the above problems by determining an adjusted volume of propane directly from a mass flow rate from a Coriolis flowmeter. Determining the adjusted volume from the mass flow rate advantageously results in more information, less maintenance, and greater accuracy. Coriolis flowmeters advantageously withstand the rigors of roadable use and do not wear out in the manner of prior meters.
One example of the invention comprises a method of delivering the propane to a customer. A transport vehicle transports the propane to the customer. The transport vehicle delivers the propane from the transport vehicle to the customer. A Coriolis flowmeter measures a mass flow rate of the propane as a transport vehicle delivers the propane to the customer. The Coriolis flowmeter determines a gross volume of the propane based on the mass flow rate of the propane and provides the gross volume. The Coriolis flowmeter also determines an adjusted volume of the propane based on the mass flow rate and a constant value, and provides the adjusted volume. The constant value corresponds to a density of the propane at a reference temperature.
Another example of the invention comprises an alternative method of delivering the propane to a customer. A transport vehicle transports the propane to the customer. The transport vehicle delivers the propane from the transport vehicle to the customer. The Coriolis flowmeter measures a mass flow rate of the propane as the transport vehicle delivers the propane to the customer. The Coriolis flowmeter determines an adjusted volume of the propane based on the mass flow rate and a constant value and provides the adjusted volume. The constant value corresponds to a density of the propane at a reference temperature.
One aspect of the invention comprises a method of delivering propane, said method comprising the steps of:
transporting said propane to a customer using a transport vehicle;
delivering said propane from said transport vehicle to said customer;
measuring a mass flow rate of said propane with a Coriolis flowmeter as said propane is being delivered from said transport vehicle to said customer;
determining a gross volume of said propane based on said mass flow rate and providing said gross volume; and
determining an adjusted volume of said propane based on said mass flow rate and a constant value and providing said adjusted volume, wherein said constant value corresponds to a density of propane at a reference temperature.
Preferably, the method further comprises the step of adjusting said constant value through a programming interface.
Preferably, adjusting said constant value comprises adjusting said constant value based on a density of propane recognized by said regulatory agency.
Preferably, adjusting said constant value comprises adjusting said constant value based on a density of propane recognized by a region of a country.
Preferably, adjusting said constant value comprises adjusting said constant value based on a density of propane recognized by a state.
Preferably, adjusting said constant value comprises adjusting said constant value based on the quality of said propane.
Preferably, wherein said gross volume is for satisfying a requirement of a regulatory agency for propane delivery.
Preferably, said reference temperature comprises approximately 60 degrees Fahrenheit.
Preferably, the method further comprises the step of generating a bill for said propane based on said adjusted volume.
Another aspect of the invention comprises a method of delivering propane, said method comprising the steps of:
transporting said propane to a customer using a transport vehicle;
delivering said propane from said transport vehicle to said customer;
measuring a mass flow rate of said propane with a Coriolis flowmeter as said propane is being delivered from said transport vehicle to said customer; and
determining an adjusted volume of said propane based on said mass flow rate and a constant value, wherein said constant value corresponds to a density of propane at a reference temperature.
Preferably, the method further comprises the step of entering said constant value.
Preferably, entering said constant value comprises entering said constant value based on a density of propane recognized by a regulatory agency.
Preferably, entering said constant value comprises entering said constant value based on a density of propane recognized by a region of a country.
Preferably, entering said constant value comprises entering said constant value based on a density of propane recognized by a state.
Preferably, entering said constant value comprises entering said constant value based on the quality of said propane.
Preferably, said reference temperature comprises approximately 60 degrees Fahrenheit.
Preferably, the method further comprises the step of generating a bill for said propane based on said adjusted volume.